High efficiency optical devices

ABSTRACT

Optical devices according to the present invention include a multilayer optical film in which at least one of the layers comprises an oriented birefringent polymer. The multilayer optical film exhibits low absorptivity and can reflect light approaching at shallow angles as well as normal to the film.

FIELD OF THE INVENTION

[0001] The present invention relates to the field of optical devices.More particularly, the present invention relates to optical devicesemploying multi-layer optical film as reflectors and/or polarizers.

BACKGROUND OF THE INVENTION

[0002] Optical devices employing reflectors are used, for example, indisplays for laptop computers, hand-held calculators, digital watchesand similar devices as well as illuminated signs, light pipes, backlightassemblies and many other devices.

[0003] Conventional reflectors, including pigmented surfaces, silveredmirrors, polished metallic or metallized surfaces, etc. suffer from anumber of disadvantages in many applications. The conventionalreflectors suffer from relatively high absorbance of light incident ontheir surfaces, typically absorbing about 4-10% of the light incident onthem. As a result, the amount of light remaining after each reflectionis less than that initially provided. In devices in which multiplereflections are encountered, the overall output of the optical devicecan be substantially limited. In addition, many of the conventionalreflectors are too bulky and/or heavy for many of the applications,particularly in laptop computer displays and other portable devices.

[0004] Many optical devices use polarizers, either alone or incombination with reflectors, to provide light having substantially oneplane of polarization. Polarized light is especially useful inconjunction with liquid crystal (LC) displays used in many portabledevices such as laptop computers and watches, because the LC displaysrely on polarized light passing through the LC to display information toa viewer.

[0005] Polarizers can be generally categorized as either absorptive orreflective. Typical absorptive polarizers are oriented dyed polymerfilms, while typical reflective polarizers are tilted thin filmpolarizers, also known as MacNeille polarizers. Absorptive polarizersdo, of course, contribute to the absorptive losses of optical devices inwhich they are used, thereby limiting the output of those devices.

[0006] The absorptive losses of known reflectors and polarizers becomemuch more important when the optical devices are used with a brightnessenhancement film such as micro-replicated brightness enhancement film orany other type of reflective polarizer which causes light to typicallytravel through several reflections, thereby amplifying absorptive losseswith every reflection. In the highest gain configurations, for, e.g., asingle sheet of brightness enhancement film in combination with areflective polarizer and back reflector, or two sheets of orthogonallycrossed sheets of brightness enhancement film, the effective absorptivelosses can reduce the total potential light output of an optical displayby 10-30%.

[0007] This principle of absorptive losses also applies to opticaldevices employing non-totally internally reflecting surfaces. Oneexample is an optical wedge in which light is directed into a structurehaving converging reflective surfaces. Optical wedges will typicallyreflect light many times before it exits the device. With eachreflection, however, some of the light which entered the wedge isabsorbed by conventional reflectors. As a result, the amount of lightexiting the device is typically substantially less than the lightentering the device.

[0008] Another optical device typically employing reflective surfaces isan illuminated sign which relies on a finite number of light sources andmultiple reflections within an optical cavity to disperse the light toilluminate the surface of a sign in a generally uniform manner. Toovercome the problems associated with absorptive losses, many signstypically employ numerous light sources, thereby increasing the cost tomanufacture and operate the signs.

[0009] Yet another optical device which is limited by absorption lossesis a light pipe in which light enters the pipe and is reflected alongits length numerous times before exiting at a desired location. Eachreflection results in some absorption when conventional reflectors areused, thereby limiting throughput of the light pipe.

[0010] To overcome some of the problems of weight, bulk and absorptionof conventional reflectors, multi-layered polymer films have been usedto reflect and/or polarize light. Such polymeric films are, however,subject to a number of other disadvantages including iridescence, aswell as poor reflectivity when off-axis light approaches the surface ofthe film. The off-axis light is typically transmitted through the films,rather than being reflected, thereby resulting in transmissive lossesrather than absorptive losses. Whether light is lost through absorptionor transmission, however, the output of the optical device is limited.

[0011] Other problems with known multi-layer polymer films used toprovide reflectors and/or polarizers is that the materials and methodsused to manufacture the films presents serious problems due to pooroptical transmission, extrudibility, and high costs.

SUMMARY OF THE INVENTION

[0012] Optical devices according to the present invention include amultilayer optical film. Optical devices incorporating multilayeroptical film according to the present invention enjoy many advantagesdue to the low absorptivity of the film and its ability to reflect lightapproaching at shallow angles as well as normal to the film.

[0013] In those situations where complete reflectivity is desired,optical devices employing a multilayer optical film according to thepresent invention can reflect over 99% of the light striking the surfaceof the film.

[0014] If a reflective polarizer is desired, the optical devices can beconstructed with a multilayer optical film which transmits a significantamount of light having one plane of polarization while reflecting asignificant amount of light having an orthogonally orientedpolarization. A further advantage is that the relative percentages oftransmitted/reflected light can be largely controlled by the multilayeroptical film used in the present invention.

[0015] As a result of the unique properties of the multilayer opticalfilm, optical devices according to the present invention are highlyefficient at reflecting and transporting light and/or transmitting lightof one polarization, whether the light is incident normal to the filmsurface or off-axis.

[0016] Another advantage of optical devices employing multilayer opticalfilm according to the present invention which rely on reflection totransport light is that the devices need not have symmetry to reduce thenumber of reflections needed to transmit light due to the lowabsorptivity of the multilayer optical film.

[0017] Yet another advantage of optical devices employing multilayeroptical films according to the present invention is their relatively lowweight as compared to many conventional reflectors and/or polarizers.

[0018] Still another advantage of optical devices employing multilayeroptical films according to the present invention is that because thefilm is relatively thin as compared to many conventional reflectorsand/or polarizers, the optical devices can be manufactured to occupylimited space in a system employing the device.

[0019] Additional features and advantages of optical devices accordingto the present invention will be apparent upon reading the detaileddescription of illustrative embodiments below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIGS. 1a and 1 b are diagrammatical views of the multilayeroptical film of the present invention.

[0021]FIG. 2 depicts a two layer stack of films forming a singleinterface.

[0022] FIGS. 3-6, 7A and 7B depict the optical performance of multilayeroptical films described in Examples 1-5.

[0023]FIG. 8 is a graphical representation illustrating the relationshipbetween the number of reflections experienced by a ray of light (x-axis)as compared to the relative intensity of the light ray (y-axis) forreflective surfaces made of multilayer optical film and a standardreflector.

[0024]FIG. 9 is schematic cross-sectional diagram of an alternateoptical device according to the present invention.

[0025]FIG. 10 is a perspective view of the optical device of FIG. 9 inwhich at least one surface of the device is intended to display amessage.

[0026]FIG. 11 is a schematic cross-sectional diagram of a convergingwedge optical device according to the present invention.

[0027]FIG. 12 is a schematic cross-sectional diagram of a divergingwedge optical device according to the present invention.

[0028]FIG. 13 is a schematic cross-sectional diagram of a light pipeemploying multilayer optical films according to the present invention.

[0029]FIG. 14 is a schematic cross-sectional diagram of the device ofFIG. 13, taken along a plane transverse to the longitudinal axis of thelight pipe.

[0030]FIG. 15 is a perspective view of one illustrative optical deviceconstructed using multilayer optical films according to the presentinvention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS OF THE INVENTION

[0031] I. Multilayer Optical Film

[0032] The optical devices described herein rely on the unique andadvantageous properties of multilayer optical films. The advantages,characteristics and manufacturing of such films are most completelydescribed in the above mentioned copending and commonly assigned U.S.patent application Ser. No. 08/402,041, filed Mar. 10, 1995, titledOPTICAL FILM, which is incorporated herein by reference. The multilayeroptical film is useful, for example, as highly efficient mirrors and/orpolarizers. A relatively brief description of the properties andcharacteristics of the multilayer optical film is presented belowfollowed by a description of illustrative embodiments of backlightsystems using the multilayer optical film according to the presentinvention.

[0033] Multilayer optical films as used in conjunction with the presentinvention exhibit relatively low absorption of incident light, as wellas high reflectivity for off-axis as well as normal light rays. Theseproperties generally hold whether the films are used for pure reflectionor reflective polarization of light. The unique properties andadvantages of the multi-layer optical film provides an opportunity todesign highly efficient backlight systems which exhibit low absorptionlosses when compared to known backlight systems.

[0034] An exemplary multilayer optical film of the present invention asillustrated in FIGS. 1A and 1B includes a multilayer stack 10 havingalternating layers of at least two materials 12 and 14. At least one ofthe materials has the property of stress induced birefringence, suchthat the index of refraction (n) of the material is affected by thestretching process. FIG. 1A shows an exemplary multilayer stack beforethe stretching process in Which both materials have the same index ofrefraction. Light ray 13 experiences no index of refraction change andpasses through the stack. In FIG. 1B, the same stack has been stretched,thus increasing the index of refraction of material 12. The differencein refractive index at each boundary between layers will cause part ofray 15 to be reflected. By stretching the multilayer stack over a rangeof uniaxial to biaxial orientation, a film is created with a range ofreflectivities for differently oriented plane-polarized incident light.The multilayer stack can thus be made useful as reflective polarizers ormirrors.

[0035] Multilayer optical films constructed according to the presentinvention exhibit a Brewster angle (the angle at which reflectance goesto zero for light incident at any of the layer interfaces) which is verylarge or is nonexistent. In contrast, known multilayer polymer filmsexhibit relatively small Brewster angles at layer interfaces, resultingin transmission of light and/or undesirable iridescence. The multilayeroptical films according to the present invention, however, allow for theconstruction of mirrors and polarizers whose reflectivity for ppolarized light decrease slowly with angle of incidence, are independentof angle of incidence, or increase with angle of incidence away from thenormal. As a result, multilayer stacks having high reflectivity for boths and p polarized light over a wide bandwidth, and over a wide range ofangles can be achieved.

[0036]FIG. 2 shows two layers of a multilayer stack, and indicates thethree dimensional indices of refraction for each layer. The indices ofrefraction for each layer are n1x, n1y, and n1z for layer 102, and n2x,n2y, and n2z for layer 104. The relationships between the indices ofrefraction in each film layer to each other and to those of the otherlayers in the film stack determine the reflectance behavior of themultilayer stack at any angle of incidence, from any azimuthaldirection. The principles and design considerations described in U.S.patent application Ser. No. 08/402,041 can be applied to createmultilayer stacks having the desired optical effects for a wide varietyof circumstances and applications. The indices of refraction of thelayers in the multilayer stack can be manipulated and tailored toproduce the desired optical properties.

[0037] Referring again to FIG. 1B, the multilayer stack 10 can includetens, hundreds or thousands of layers, and each layer can be made fromany of a number of different materials. The characteristics whichdetermine the choice of materials for a particular stack depend upon thedesired optical performance of the stack. The stack can contain as manymaterials as there are layers in the stack. For ease of manufacture,preferred optical thin film stacks contain only a few differentmaterials.

[0038] The boundaries between the materials, or chemically identicalmaterials with different physical properties, can be abrupt or gradual.Except for some simple cases with analytical solutions, analysis of thelatter type of stratified media with continuously varying index isusually treated as a much larger number of thinner uniform layers havingabrupt boundaries but with only a small change in properties betweenadjacent layers.

[0039] The preferred multilayer stack is comprised of low/high indexpairs of film layers, wherein each low/high index pair of layers has acombined optical thickness of ½ the center wavelength of the band it isdesigned to reflect. Stacks of such films are commonly referred to asquarterwave stacks. For multilayer optical films concerned with thevisible and the near infrared wavelengths, a quarterwave stack designresults in each of the layers in the multilayer stack having an averagethickness of not more than 0.5 microns.

[0040] In those applications where reflective films (e.g. mirrors) aredesired, the desired average transmission for light of each polarizationand plane of incidence generally depends upon the intended use of thereflective film. One way to produce a multilayer mirror film is tobiaxially stretch a multilayer stack which contains a birefringentmaterial as the high index layer of the low/high index pair. For a highefficiency reflective film, average transmission along each stretchdirection at normal incidence over the visible spectrum (400-700 nm) isdesirably less than 10% (reflectance greater than 90%), preferably lessthan 5% (reflectance greater than 95%), more preferably less than 2%(reflectance greater than 98%), and even more preferably less than 1%(reflectance greater than 99%). The average transmission at 60 degreesfrom the normal from 400-700 nm is desirably less than 20% (reflectancegreater than 80%), preferably less than 10% (reflectance greater than90%), more preferably less than 5% (reflectance greater than 95%), andeven more preferably less than 2% (reflectance greater than 98%), andeven more preferably less than 1% (reflectance greater than 99%).

[0041] In addition, asymmetric reflective films may be desirable forcertain applications. In that case, average transmission along onestretch direction may be desirably less than, for example, 50%, whilethe average transmission along the other stretch direction may bedesirably less than, for example 20%, over a bandwidth of, for example,the visible spectrum (400-700 nm), or over the visible spectrum and intothe near infrared (e.g, 400-850 nm).

[0042] Multilayer optical films can also be designed to operate asreflective polarizers. One way to produce a multilayer reflectivepolarizer is to uniaxially stretch a multilayer stack which contains abirefringent material as the high index layer of the low/high indexpair. The resulting reflective polarizers have high reflectivity forlight with its plane of polarization parallel to one axis (in thestretch direction) for a broad range of angles of incidence, andsimultaneously have low reflectivity and high transmissivity for lightwith its plane of polarization parallel to the other axis (in thenon-stretch direction) for a broad range of angles of incidence. Bycontrolling the three indices of refraction of each film, nx, ny and nz,the desired polarizer behavior can be obtained.

[0043] For many applications, the ideal reflecting polarizer has highreflectance along one axis (the so-called extinction axis) and zeroreflectance along the other (the so-called transmission axis), at allangles of incidence. For the transmission axis of a polarizer, itgenerally desirable to maximize transmission of light polarized in thedirection of the transmission axis over the bandwidth of interest andalso over the range of angles of interest.

[0044] The average transmission at normal incidence for a polarizer inthe transmission axis across the visible spectrum (400-700 nm for abandwidth of 300 nm) is desirably at least 50%, preferably at least 70%,more preferably at least 85%, and even more preferably at least 90%. Theaverage transmission at 60 degrees from the normal (measured along thetransmission axis for p-polarized light) for a polarizer from 400-700 nmis desirably at least 50%, preferably at least 70%, more preferably atleast 80%, and even more preferably at least 90%.

[0045] The average transmission for a multilayer reflective polarizer atnormal incidence for light polarized in the direction of the extinctionaxis across the visible spectrum (400-700 nm for a bandwidth of 300 nm)is desirably at less than 50%, preferably less than 30%, more preferablyless than 15%, and even more preferably less than 5%. The averagetransmission at 60 degrees from the normal (measured along thetransmission axis for p-polarized light) for a polarizer for lightpolarized in the direction of the extinction axis from 400-700 nm isdesirably less than 50%, preferably less than 30%, more preferably lessthan 15%, and even more preferably less than 5%.

[0046] For certain applications, high reflectivity for p-polarized lightwith its plane of polarization parallel to the transmission axis atoff-normal angles are preferred. The average reflectivity for lightpolarized along the transmission axis should be more than 20% at anangle of at least 30 degrees from the normal.

[0047] In addition, although reflective polarizing films and asymmetricreflective films are discussed separately herein, it should beunderstood that two or more of such films could be provided to reflectsubstantially all light incident on them (provided they are properlyoriented with respect to each other to do so). This construction istypically desired when the multilayer optical film is used as areflector in a backlight system according to the present invention.

[0048] If some reflectivity occurs along the transmission axis, theefficiency of the polarizer at off-normal angles may be reduced. If thereflectivity along the transmission axis is different for variouswavelengths, color may be introduced into the transmitted light. One wayto measure the color is to determine the root mean square (RMS) value ofthe transmissivity at a selected angle or angles over the wavelengthrange of interest. The % RMS color, C_(RMS), can be determined accordingto the equation:$C_{RMS} = \frac{\int_{\lambda \quad 1}^{\lambda 2}{\left( \left( {T - \overset{\_}{T}} \right)^{2} \right)^{1/2}{\lambda}}}{\overset{\_}{T}}$

[0049] where the range λ1 to λ2 is the wavelength range, or bandwidth,of interest, T is the transmissivity along the transmission axis, and{overscore (T)} is the average transmissivity along the transmissionaxis in the wavelength range of interest. For applications where a lowcolor polarizer is desirable, the % RMS color should be less than 10%,preferably less than 8%, more preferably less than 3.5%, and even morepreferably less than 2% at an angle of at least 30 degrees from thenormal, preferably at least 45 degrees from the normal, and even morepreferably at least 60 degrees from the normal.

[0050] Preferably, a reflective polarizer combines the desired % RMScolor along the transmission axis for the particular application withthe desired amount of reflectivity along the extinction axis across thebandwidth of interest. For polarizers having a bandwidth in the visiblerange (400-700 nm, or a bandwidth of 300 nm), average transmission alongthe extinction axis at normal incidence is desirably less than 40%, moredesirably less than 25%, preferably less than 15%, more preferably lessthan 5% and even more preferably less than 3%.

[0051] Materials Selection and Processing

[0052] With the design considerations described in the above mentionedU.S. patent application Ser. No. 08/402,041, one of ordinary skill willreadily appreciate that a wide variety of materials can be used to formmultilayer reflective films or polarizers according to the inventionwhen processed under conditions selected to yield the desired refractiveindex relationships. The desired refractive index relationships can beachieved in a variety of ways, including stretching during or after filmformation (e.g., in the case of organic polymers), extruding (e.g., inthe case of liquid crystalline materials), or coating. In addition, itis preferred that the two materials have similar rheological properties(e.g., melt viscosities) such that they can be co-extruded.

[0053] In general, appropriate combinations may be achieved byselecting, as the first material, a crystalline or semi-crystallinematerial, preferably a polymer. The second material, in turn, may becrystalline, semi-crystalline, or amorphous. The second material mayhave a birefringence opposite of the first material. Or, the secondmaterial may have no birefringence, or less birefringence than the firstmaterial.

[0054] Specific examples of suitable materials include polyethylenenaphthalate (PEN) and isomers thereof (e.g., 2,6-, 1,4-, 1,5-, 2,7-, and2,3-PEN), polyalkylene terephthalates (e.g., polyethylene terephthalate,polybutylene terephthalate, and poly-1,4-cyclohexanedimethyleneterephthalate), polyimides (e.g., polyacrylic imides), polyetherimides,atactic polystyrene, polycarbonates, polymethacrylates (e.g.,polyisobutyl methacrylate, polypropylmethacrylate,polyethylmethacrylate, and polymethylmethacrylate), polyacrylates (e.g.,polybutylacrylate and polymethylacrylate), syndiotactic polystyrene(sPS), syndiotactic poly-alpha-methyl styrene, syndiotacticpolydichlorostyrene, copolymers and blends of any of these polystyrenes,cellulose derivatives (e.g., ethyl cellulose, cellulose acetate,cellulose propionate, cellulose acetate butyrate, and cellulosenitrate), polyalkylene polymers (e.g., polyethylene, polypropylene,polybutylene, polyisobutylene, and poly(4-methyl)pentene), fluorinatedpolymers (e.g., perfluoroalkoxy resins, polytetrafluoroethylene,fluorinated ethylene-propylene copolymers, polyvinylidene fluoride, andpolychlorotrifluoroethylene), chlorinated polymers (e.g., polyvinylidenechloride and polyvinylchloride), polysulfones, polyethersulfones,polyacrylonitrile, polyamides, silicone resins, epoxy resins,polyvinylacetate, polyether-amides, ionomeric resins, elastomers (e.g.,polybutadiene, polyisoprene, and neoprene), and polyurethanes. Alsosuitable are copolymers, e.g., copolymers of PEN (e.g., copolymers of2,6-, 1,4-, 1,5-, 2,7-, and/or 2,3-naphthalene dicarboxylic acid, oresters thereof, with (a) terephthalic acid, or esters thereof; (b)isophthalic acid, or esters thereof; (c) phthalic acid, or estersthereof; (d) alkane glycols; (e) cycloalkane glycols (e.g., cyclohexanedimethane diol); (f) alkane dicarboxylic acids; and/or (g) cycloalkanedicarboxylic acids (e.g., cyclohexane dicarboxylic acid)), copolymers ofpolyalkylene terephthalates (e.g., copolymers of terephthalic acid, oresters thereof, with (a) naphthalene dicarboxylic acid, or estersthereof; (b) isophthalic acid, or esters thereof; (c) phthalic acid, oresters thereof; (d) alkane glycols; (e) cycloalkane glycols (e.g.,cyclohexane dimethane diol); (f) alkane dicarboxylic acids; and/or (g)cycloalkane dicarboxylic acids (e.g., cyclohexane dicarboxylic acid)),and styrene copolymers (e.g., styrene-butadiene copolymers andstyrene-acrylonitrile copolymers), 4,4′-bibenzoic acid and ethyleneglycol. In addition, each individual layer may include blends of two ormore of the above-described polymers or copolymers (e.g., blends of sPSand atactic polystyrene). The coPEN described may also be a blend ofpellets where at least one component is a polymer based on naphthalenedicarboxylic acid and other components are other polyesters orpolycarbonates, such as a PET, a PEN or a co-PEN.

[0055] Particularly preferred combinations of layers in the case ofpolarizers include PEN/co-PEN, polyethylene terephthalate (PET)/co-PEN,PEN/sPS, PET/sPS, PEN/Eastar, and PET/Eastar, where “co-PEN” refers to acopolymer or blend based upon naphthalene dicarboxylic acid (asdescribed above) and Eastar is polycyclohexanedimethylene terephthalatecommercially available from Eastman Chemical Co.

[0056] Particularly preferred combinations of layers in the case ofreflective films include PET/Ecdel, PEN/Ecdel, PEN/sPS, PEN/THV,PEN/co-PET, and PET/sPS, where “co-PET” refers to a copolymer or blendbased upon terephthalic acid (as described above), Ecdel is athermoplastic polyester commercially available from Eastman ChemicalCo., and THV is a fluoropolymer commercially available from MinnesotaMining and Manufacturing Company, St. Paul, Minn.

[0057] The number of layers in the film is selected to achieve thedesired optical properties using the minimum number of layers forreasons of film thickness, flexibility and economy. In the case of bothpolarizers and reflective films, the number of layers is preferably lessthan 10,000, more preferably less than 5,000, and even more preferablyless than 2,000.

[0058] As discussed above, the ability to achieve the desiredrelationships among the various indices of refraction (and thus theoptical properties of the multilayer film) is influenced by theprocessing conditions used to prepare the multilayer film. In the caseof organic polymers which can be oriented by stretching, the films aregenerally prepared by co-extruding the individual polymers to form amultilayer film and then orienting the film by stretching at a selectedtemperature, optionally followed by heat-setting at a selectedtemperature. Alternatively, the extrusion and orientation steps may beperformed simultaneously. In the case of polarizers, the film isstretched substantially in one direction (uniaxial orientation), whilein the case of reflective films the film is stretched substantially intwo directions (biaxial orientation).

[0059] The film may be allowed to dimensionally relax in thecross-stretch direction from the natural reduction in cross-stretch(equal to the square root of the stretch ratio); it may simply beconstrained to limit any substantial change in cross-stretch dimension;or it may be actively stretched in the cross-stretch dimension. The filmmay be stretched in the machine direction, as with a length orienter, orin width using a tenter.

[0060] The pre-stretch temperature, stretch temperature, stretch rate,stretch ratio, heat set temperature, heat set time, heat set relaxation,and cross-stretch relaxation are selected to yield a multilayer filmhaving the desired refractive index relationship. These variables areinter-dependent; thus, for example, a relatively low stretch rate couldbe used if coupled with, e.g., a relatively low stretch temperature. Itwill be apparent to one of ordinary skill how to select the appropriatecombination of these variables to achieve the desired multilayer film.In general, however, a stretch ratios in the range from 1:2 to 1:10(more preferably 1:3 to 1:7) in the stretch direction and from 1:0.2 to1:10 (more preferably from 1:0.3 to 1:7) orthogonal to the stretchdirection is preferred.

[0061] Suitable multilayer films may also be prepared using techniquessuch as spin coating (e.g., as described in Boese et al., J. Polym.Sci.: Part B, 30:1321 (1992) for birefringent polyimides, and vacuumdeposition (e.g., as described by Zang et. al., Appl. Phys. Letters,59:823 (1991) for crystalline organic compounds; the latter technique isparticularly useful for certain combinations of crystalline organiccompounds and inorganic materials.

[0062] Exemplary multilayer reflective mirror films and multilayerreflective polarizers will now be described in the following examples.

EXAMPLE 1 PEN:THV 500, 449 Mirror

[0063] A coextruded film containing 449 layers was made by extruding thecast web in one operation and later orienting the film in a laboratoryfilm-stretching apparatus. A Polyethylene naphthalate (PEN) with anIntrinsic Viscosity of 0.53 dl/g (60 wt. % phenol/40 wt. %dichlorobenzene) was delivered by one extruder at a rate of 56 poundsper hour and THV 500 (a fluoropolymer available from Minnesota Miningand Manufacturing Company) was delivered by another extruder at a rateof 11 pounds per hour. The PEN was on the skin layers and 50% of the PENwas present in the two skin layers. The feedblock method was used togenerate 57 layers which was passed through three multipliers producingan extrudate of 449 layers. The cast web was 20 mils thick and 12 incheswide. The web was later biaxially oriented using a laboratory stretchingdevice that uses a pantograph to grip a square section of film andsimultaneously stretch it in both directions at a uniform rate. A 7.46cm square of web was loaded into the stretcher at about 100° C. andheated to 140° C. in 60 seconds. Stretching then commenced at 10%/sec(based on original dimensions) until the sample was stretched to about3.5×3.5. Immediately after the stretching the sample was cooled byblowing room temperature air at it.

[0064]FIG. 3 shows the transmission of this multilayer film. Curve (a)shows the response at normal incidence, while curve (b) shows theresponse at 60 degrees for p-polarized light.

EXAMPLE 2 PEN:PMMA, 601, Mirror

[0065] A coextruded film containing 601 layers was made on a sequentialflat-film-making line via a coextrusion process. PolyethyleneNaphthalate (PEN) with an Intrinsic Viscosity of 0.57 dl/g (60 wt. %phenol/40 wt. % dichlorobenzene) was delivered by extruder A at a rateof 114 pounds per hour with 64 pounds per hour going to the feedblockand the rest going to skin layers described below. PMMA (CP-82 from ICIof Americas) was delivered by extruder B at a rate of 61 pounds per hourwith all of it going to the feedblock. PEN was on the skin layers of thefeedblock. The feedblock method was used to generate 151 layers usingthe feedblock such as those described in U.S. Pat. No. 3,801,429, afterthe feedblock two symmetric skin layers were coextruded using extruder Cmetering about 30 pounds per hour of the same type of PEN delivered byextruder A. This extrudate passed through two multipliers producing anextrudate of about 601 layers. U.S. Pat. No. 3,565,985 describes similarcoextrusion multipliers. The extrudate passed through another devicethat coextruded skin layers at a total rate of 50 pounds per hour of PENfrom extruder A. The web was length oriented to a draw ratio of about3.2 with the web temperature at about 280° F. The film was subsequentlypreheated to about 310° F. in about 38 seconds and drawn in thetransverse direction to a draw ratio of about 4.5 at a rate of about 11%per second. The film was then heat-set at 440° F. with no relaxationallowed. The finished film thickness was about 3 mil.

[0066] As seen in FIG. 4, curve (a), the bandwidth at normal incidenceis about 350 nm with an average in-band extinction of greater than 99%.The amount of optical absorption is difficult to measure because of itslow value, but is less than 1%. At an incidence angle of 50° from thenormal both s (curve (b)) and p-polarized (curve (c)) light showedsimilar extinctions, and the bands were shifted to shorter wavelengthsas expected. The red band-edge for s-polarized light is not shifted tothe blue as much as for p-polarized light due to the expected largerbandwidth for s-polarized light, an due to the lower index seen by thep-polarized light in the PEN layers.

EXAMPLE 3 PEN:PCTG, 449, Polarizer

[0067] A coextruded film containing 481 layers was made by extruding thecast web in one operation and later orienting the film in a laboratoryfilm-stretching apparatus. The feedblock method was used with a 61 layerfeedblock and three (2×) multipliers. Thick skin layers were addedbetween the final multiplier and the die. Polyethylene naphthalate (PEN)with an intrinsic viscosity of 0.47 dl/g (60 wt. % phenol/40 wt. %dichlorobenzene) was delivered to the feedblock by one extruder at arate of 25.0 pounds per hour. Glycol modified polyethylene dimethylcyclohexane terephthalate (PCTG 5445 from Eastman) was delivered byanother extruder at a rate of 25.0 pounds per hour. Another stream ofPEN from the above extruder was added as skin layers after themultipliers at a rate of 25.0 pounds per hour. The cast web was 0.007inches thick and 12 inches wide. The web was layer uniaxially orientedusing a laboratory stretching device that uses a pantograph to grip asection of film and stretch it in one direction at a uniform rate whileit is allowed to freely relax in the other direction. The sample of webloaded was about 5.40 cm wide (the unconstrained direction) and 7.45 cmlong between the grippers of the pantograph. The web was loaded into thestretcher at about 100° C. and heated to 135° C. for 45 seconds.Stretching was then commenced at 20%/second (based on originaldimensions) until the sample was stretched to about 6:1 (based ongripper to gripper measurements). Immediately after stretching, thesample was cooled by blowing room temperature air at it. In the center,the sample was found to relax by a factor of 2.0.

[0068]FIG. 5 shows the transmission of this multilayer film where curvea shows transmission of light polarized in the non-stretch direction atnormal incidence, curve b shows transmission of p-polarized lightpolarized in the non-stretched direction at 60° incidence, and curve cshows the transmission of light polarized in the stretch direction atnormal incidence. Average transmission for curve a from 400-700 nm is89.7%, average transmission for curve b from 400-700 nm is 96.9%, andaverage transmission for curve c from 400-700 nm is 4.0%. % RMS colorfor curve a is 1.05%, and % RMS color for curve b is 1.44%.

EXAMPLE 4 PEN:CoPEN, 601, Polarizer

[0069] A coextruded film containing 601 layers was made on a sequentialflat-film-making line via a coextrusion process. A Polyethylenenaphthalate (PEN) with an intrinsic viscosity of 0.54 dl/g (60 wt %Phenol plus 40 wt % dichlorobenzene) was delivered by on extruder at arate of 75 pounds per hour and the coPEN was delivered by anotherextruder at 65 pounds per hour. The coPEN was a copolymer of 70 mole %2,6 naphthalene dicarboxylate methyl ester, 15% dimethyl isophthalateand 15% dimethyl terephthalate with ethylene glycol. The feedblockmethod was used to generate 151 layers. The feedblock was designed toproduce a stack of films having a thickness gradient from top to bottom,with a thickness ratio of 1.22 from the thinnest layers to the thickestlayers. The PEN skin layers were coextruded on the outside of theoptical stack with a total thickness of 8% of the coextruded layers. Theoptical stack was multiplied by two sequential multipliers. The nominalmultiplication ratio of the multipliers were 1.2 and 1.27, respectively.The film was subsequently preheated to 310° F. in about 40 seconds anddrawn in the transverse direction to a draw ratio of about 5.0 at a rateof 6% per second. The finished film thickness was about 2 mils.

[0070]FIG. 6 shows the transmission for this multilayer film. Curve ashows transmission of light polarized in the non-stretch direction atnormal incidence, curve b shows transmission of p-polarized light at 60°incidence, and curve c shows transmission of light polarized in thestretch direction at normal incidence. Note the very high transmissionof p-polarized light in the non-stretch direction at both normal and 60°incidence (80-100%). Also note the very high reflectance of lightpolarized in the stretched direction in the visible range (400-700 nm)shown by curve c. Reflectance is nearly 99% between 500 and 650 nm.

EXAMPLE 5 PEN:sPS, 481, Polarizer

[0071] A 481 layer multilayer film was made from a polyethylenenaphthalate (PEN) with an intrinsic viscosity of 0.56 dl/g measured in60 wt. % phenol and 40 wt % dichlorobenzene purchased from EastmanChemicals and a syndiotactic polystyrene (sPS) homopolymer (weightaverage molecular weight=200,000 Daltons, sampled from Dow Corporation).The PEN was on the outer layers and was extruded at 26 pounds per hourand the sPS at 23 pounds per hour. The feedblock used produced 61 layerswith each of the 61 being approximately the same thickness. After thefeedblock three (2×) multipliers were used. Equal thickness skin layerscontaining the same PEN fed to the feedblock were added after the finalmultiplier at a total rate of 22 pounds per hour. The web was extrudedthrough a 12″ wide die to a thickness of about 0.011 inches (0.276 mm).The extrusion temperature was 290° C.

[0072] This web was stored at ambient conditions for nine days and thenuniaxially oriented on a tenter. The film was preheated to about 320° F.(160° C.) in about 25 seconds and drawn in the transverse direction to adraw ratio of about 6:1 at a rate of about 28% per second. No relaxationwas allowed in the stretched direction. The finished film thickness wasabout 0.0018 inches (0.046 mm).

[0073]FIG. 7 shows the optical performance of this PEN:sPS reflectivepolarizer containing 481 layers. Curve a shows transmission of lightpolarized in the non-stretch direction at normal incidence, curve bshows transmission of p-polarized light at 60° incidence, and curve cshows transmission of light polarized in the stretch direction at normalincidence. Note the very high transmission of p-polarized light at bothnormal and 60° incidence. Average transmission for curve a over 400-700nm is 86.2%, the average transmission for curve b over 400-700 nm is79.7%. Also note the very high reflectance of light polarized in thestretched direction in the visible range (400-700 nm) shown by curve c.The film has an average transmission of 1.6% for curve c between 400 and700 nm. The % RMS color for curve a is 3.2%, while the % RMS color forcurve b is 18.2%.

EXAMPLE 6 PEN:coPEN, 603, Polarizer

[0074] A reflecting polarizer comprising 603 layers was made on asequential flat-film making line via a coextrusion process. Apolyethylene naphthalate (PEN) with an intrinsic viscosity of 0.47 dl/g(in 60 wt % phenol plus 40 wt % dichlorobenzene) was delivered by anextruder at a rate of 83 pounds (38 kg) per hour and the CoPEN wasdelivered by another extruder at 75 pounds (34 kg) per hour. The CoPENwas a copolymer of 70 mole %, 2,6 naphthalene dicarboxylate methylester, 15 mole % dimethyl terephthalate, and 15 mole % dimethylisophthalate with ethylene glycol. The feedblock method was used togenerate 151 layers. The feedblock was designed to produce a stack offilms having a thickness gradient from top to bottom, with a thicknessratio of 1.22 from the thinnest layers to the thickest layers. Thisoptical stack was multiplied by two sequential multipliers. The nominalmultiplication ratio of the multipliers was 1.2 and 1.4, respectively.Between the final multiplier and the die, skin layers were addedcomposed of the same CoPEN described above, delivered by a thirdextruder at a total rate of 106 pounds (48 kg) per hour. The film wassubsequently preheated to 300° F. (150° C.) in about 30 seconds anddrawn in the transverse direction to a draw ratio of approximately 6 atan initial rate of about 20% per second. The finished film thickness wasapproximately 0.0035 inch (0.089 mm).

[0075]FIG. 7B shows the optical performance of the polarizer of Example6. Curve a shows transmission of light polarized in the non-stretchdirection at normal incidence, curve b shows transmission of p-polarizedlight in the nonstretch direciton at 50 degree angle of incidence, andcurve c shows transmission of light polarized in the stretch directionat normal incidence. Note the very high transmission of light polarizedin the non-stretch direction. Average transmission for curve a over400-700 nm is 87%. Also note the very high reflectance of lightpolarized in the stretched direction in the visible range (400-700 nm)shown by curve c. The film has an average transmission of 2.5% for curvec between 400 and 700 nm. The % RMS color for curve b is 5%.

[0076] II. Optical Devices Using Multilayer Optical Films

[0077] Optical devices according to the present invention use multilayeroptical films to polarize and/or reflect light. The advantages of usingmultilayer optical film in optical devices involving reflection of lightare graphically illustrated in FIG. 8. Curve a shows the totalreflectivity as a function of the number of reflections for conventionreflector that has 96% reflectivity (i.e., about 4% of the light isabsorbed at each reflection). As shown by curve a, the intensity oflight which has been reflected decreases significantly after arelatively low number of reflections when the surface reflecting thelight absorbs only about 4% of the light. In contrast, curve b shows thetotal reflectivity for a multilayer mirror film having a reflectivity ofabout 99.4%. Curve b clearly shows a much smaller decrease in totalreflectivity. The difference becomes especially pronounced after only2-4 reflections.

[0078] For example, for five reflections, the intensity of light isabout 97% for light reflected from multilayer optical films according tothe present invention, while the intensity drops to about 81.5% forlight reflected from a conventional reflector which is only about 3.5%less efficient. Although it is difficult to determine the average numberof reflections experienced by light in a backlight system, the number ofreflections can be expected to increase as aspect ratio (defined morecompletely below) increase in any given backlight system. Thoseincreased reflections would cause a significant loss in efficiency forbacklight systems using conventional reflectors which would not beexperienced in backlight systems employing multilayer optical filmreflectors according to the present invention.

[0079] The practical value of this characteristic is that the efficiencyof the present optical device is greatly enhanced as compared to systemsemploying conventional reflectors. Stated another way, the number ofacceptable reflections for a given light ray in optical devicesemploying multilayer optical film according to the present invention canbe significantly increased without substantially impairing the overalloutput of the device as compared to optical devices employing knownreflectors/polarizers. This means that the present optical devices canbe used to transmit and transport light over greater distances withbetter efficiency than presently known conventional reflectors.

[0080] Optical devices which incorporate the multilayer optical film canbe most generally described as devices in which at least a portion ofthe light entering and/or exiting the device is reflected from anoptical surface comprising the multilayer optical film. For the purposeof this invention, an “optical surface” will be defined as a surface,planar or otherwise, which reflects at least a portion of randomlypolarized light incident upon it. More preferably, at least a portion ofthe light traveling through the optical devices will be reflected froman optical surface more than once, thereby exploiting the advantages ofthe multilayer optical film.

[0081] A subset of optical devices incorporating multilayer optical filmaccording to the present invention will comprise two or more opticalsurfaces and can generally be categorized into devices in which theoptical surfaces are arranged in a parallel or a non-parallel opposingarrangement.

[0082] Optical devices with substantially parallel optical surfacesinclude, but are not limited to: light pipes, light boxes, rectangularlight guides, etc. For those devices designed to transmit light from onelocation to another, such as a light pipe, it is desirable that theoptical surfaces absorb and transmit a minimal amount of light incidentupon them while reflecting substantially all of the light. In otherdevices such as light boxes and light guides, it may be desirable todeliver light to a selected area using generally reflective opticalsurfaces and to then allow for transmission of light out of the devicein a known, predetermined manner. In such devices, it may be desirableto provide a portion of the optical surface as partially reflective toallow light to exit the device in a predetermined manner. Examples ofsuch devices will be described more completely below.

[0083] Another class of optical devices which include two or morereflective optical surfaces are devices in which the reflective opticalsurfaces converge towards each other as distance from a light source (orpoint of entry into the device) increases. This construction isespecially useful in optical devices where it is desired to return lightemitted from an optical source towards the direction from which thelight entered the device. Optical devices with converging reflectiveoptical surfaces will typically reflect a majority of light in adirection generally towards the source of the light.

[0084] Yet another class of optical devices which include two or moreoptical surfaces are devices in which the reflective optical surfacesdiverge as distance from a light source (or point of entry into thedevice) increases. Optical devices with diverging reflective opticalsurfaces will typically tend to collimate light. The amount and degreeof collimation will depend on the location of the light source relativeto the narrow end of the device and the rate of divergence of themultilayer reflective optical film surfaces.

[0085] In a preferred embodiment, the optical devices are hollow as thiswill tend to decrease the amount of absorption at each reflection aslight is transported by the optical devices.

[0086] In the effort to direct light towards a specific target, such asin task lighting, solar collectors, or otherwise, it may be preferredthat the diverging optical surfaces form a parabola or cone. If aparabolic shape is used, collimation is best accomplished for lightpassing through or emanating from the focal point of the parabola. Thespecifics of designing the shape of such devices will be well known tothose skilled in the art and will not be discussed herein.

[0087] Turning now to the figures in which illustrative examples ofoptical devices according to the present invention are depicted, FIGS. 9and 10 depict one illustrative optical device 110 in a cross-sectionalschematic view in FIG. 9 and a perspective view in FIG. 10. Opticaldevice 110 is commonly referred to as a light box and can besubstantially rectangular as shown or it can take any other shapedesired based on aesthetics or functional considerations. Light boxesare typically substantially enclosed volumes in which one or more lightsources are located. The volume is preferably lined with a reflectivesurface and includes either partially reflective areas or voids whichallow light to escape from the light box in a predetermined pattern ormanner.

[0088] The illustrative light box 110 depicted in FIGS. 9 and 10includes at least two opposing reflective and/or partially reflectiveoptical surfaces 112 and 114 comprised of the multilayer optical film.It is most preferred that all of the interior reflective surfaces of thelight box 110 are covered by the multilayer optical film. By using themultilayer optical film according to the present invention for all ofthe reflective surfaces within the light box 110, absorption losses canbe greatly reduced as compared to devices using conventional reflectorsand/or polarizers. In some instances, however, all or a portion ofeither or both optical surfaces 112 and 114 can be constructed fromother materials.

[0089] Where multilayer optical film is used in any optical device, itwill be understood that it can be laminated to a support (which itselfmay be transparent, opaque reflective or any combination thereof) or itcan be otherwise supported using any suitable frame or other supportstructure because in some instances the multilayer optical film itselfmay not be rigid enough to be self-supporting in an optical device suchas illustrative device 110.

[0090] The optical device 110 illustrated in FIG. 9 includes two lightsources 118 a and 118 b, referred to commonly as 118, which emit lightinto the interior of the device 110. Light emitted from the sources 118will typically reflect between surfaces 112 and 114 numerous timesbefore exiting the device 110 through a partially reflective area ortransmissive void located in surface 112, denoted by reference number130 in FIG. 10.

[0091] For illustration purposes, light rays 120 and 122 are shown asemanating from source 118 a and reflecting within the optical device 110until they exit from areas such as 130 in layer 112. In an illuminatedsign depicted as the illustrative optical device 110, areas 130 willtypically comprise advertising or other informational messages or,alternatively, may comprise a decorative display of some type. Althoughonly areas 130 are depicted as transmitting light through opticalsurface 112, it will be understood that all or any portion of bothsurfaces 112 and 114 may transmit light out of device 110.

[0092] Areas 130 which transmit light can be provided of many differentmaterials or constructions. The areas 130 can be made of multilayeroptical film or any other transmissive or partially transmissivematerials. One way to allow for light transmission through areas 130 isto provide areas in optical surface 112 which are partially reflectiveand partially transmissive. Partial reflectivity can be imparted tomultilayer optical films in areas 130 according to the present inventionby a variety of means.

[0093] In one aspect, areas 130 may comprise multi-layered optical filmwhich is uniaxially stretched to allow transmission of light having oneplane of polarization while reflecting light having a plane ofpolarization orthogonal to the transmitted light. Rays 120 a and 120 bof light as depicted in FIG. 9 illustrate such a situation in whichlight having one polarization direction is transmitted throughmulti-layered optical film 130 while light having the orthogonalpolarization direction is reflected back into optical device 110.

[0094] When areas 130 are provided from a multilayer reflectivepolarizing film, it is preferable that the optical device 110 includesome mechanism for randomizing polarization orientation of the lightreflected back into the interior of the device 110. One mechanism forrandomizing polarization orientation would be to provide a thinpigmented coating on optical surface 114 to randomize polarization andscatter light reflected from the areas 130. Another mechanism is to adda birefringent polymer film, or to have a birefringent skin layer on theMOF mirror. Any mechanism, however, by which the polarizationorientation of returned light 120 b can be modified after reflectionfrom the reflective polarizing areas 130 is desirable as it can then bereturned to areas 130 and, theoretically, a portion of the light willthen have the proper polarization orientation to allow transmissionthrough areas 130 and out of optical device 110.

[0095] Light ray 122 depicts the effect of an alternate means ofproviding for transmission of light through areas 130 in an opticaldevice 110 according to the present invention. Light ray 122 istransmitted through areas 130 without reflection through a void formedin the optical surface 112. As a result, there is no partial reflectionof light ray 122 as opposed to light ray 120 as described above. In thissituation, optical surface 112 is itself substantially completelyreflective, except for those voids in areas 130 which transmit lightwithout substantial reflection.

[0096] It will be understood that the term “void” can be used todescribe an actual physical aperture through optical surface 112 as wellas clear or transparent areas formed in the optical surface 112 which donot substantially reflect light. The number and size of multipleapertures in area 130 of optical surface 112 may be varied to controlthe amount of light transmitted through the areas 130. At one extreme,areas 130 may even constitute complete voids in optical surface 112,although large voids are typically undesirable to protect the interiorof the device 110 from debris, dust, etc.

[0097] An alternate embodiment of an optical device 110 can be providedwhere at least the areas 130 in optical surface 112 do not comprise amultilayer optical film at all, but rather comprise a different class ofpartially reflective films, such as a structured partially reflectivefilm. Exemplary micro-replicated structured partially reflective filmsare marketed as Optical Lighting Film, and Brightness Enhancement Film,available from Minnesota Mining and Manufacturing Company, St. Paul,Minn.

[0098] In those instances where a less efficient multilayer optical filmis used (i.e., some of the light incident upon the multilayer opticalfilm surfaces is lost through transmission), it may be advantageous toprovide the back surfaces of the multilayer optical film, i.e., thesurface facing the exterior of the device 110, with a thin metal orother reflective coating to reflect light that would otherwise be lostto transmission, thereby improving the reflectivity of the multilayeroptical film. It will of course, be understood that the metallic orother reflective coating may suffer from some absorption losses, but thefraction of light transmitted through the film will typically be lessthan 5% (more preferably less than 1%) of the total light incident onthe film. The metallic or other reflective coating may also be useful toreduce visible iridescence if leakage of narrow bands of wavelengthsoccurs in the multilayer optical film. In general, however, the highefficiency multilayer reflective films are preferred.

[0099] Due to the high efficiency of the multilayer optical film inreflecting light in optical devices 110, the number and intensity oflight sources 118 needed to provide uniform illumination over the areas130 can be reduced. Any optical device design can be less concernedabout the number of reflections a light ray will make within device 110before exiting as illustrated in FIG. 10 and described above.

[0100] Aspect ratio in a device 110 is typically determined by comparingthe depth of the light box, indicated as D in FIG. 10 to the length andheight of the device 110, indicated as L and H, respectively. In someinstances, aspect ratio may be the ratio of depth D as compared to thearea which is defined by the length times the height of optical device110.

[0101]FIG. 11 is a schematic cross-sectional representation of aconverging wedge optical device 210, according to the present invention,incorporating multilayer optical film. In any optical device employing aconverging wedge design, the optical surfaces 212 and 214 are arrangedin a converging relationship in which the optical surfaces convergetowards each other as distance from the opening 211 into the device 210increases. In the preferred embodiment, the surfaces 212 and 214 arecomprised of a multilayer optical film. Also, the optical device 210 ispreferably hollow to minimize absorption losses.

[0102] It will be understood that the optical device 210 could comprisetwo generally planar optical surfaces 212 and 214. One specific exampleof a converging wedge design would be a light guide used in a backlightassembly for a liquid crystal display device. Another specific exampleof an optical device represented in FIG. 11 could comprise a generallyconical device having a cross-section taken along the longitudinal axisof the device 210. In a conical device, optical surfaces 212 and 214 mayactually be portions of a continuous surface which appears discontinuousdue to the cross-sectional nature of the view in FIG. 11.

[0103] A light ray 220 is depicted as entering the optical device 210through opening 211 as shown and is reflected numerous times beforeexiting in generally the same direction from which it entered the device210. Optical surfaces 212 and 214 could be comprised of many differentmaterials. For example both surfaces 212 and 214 could be comprised ofmultilayer optical films according to the present invention and aportion or all of either or both surfaces 212 and 214 could becompletely reflective or partially reflective.

[0104] If a less efficient multilayer optical film is used forreflective optical surfaces 212 and 214 and it is desired that bothsurfaces prevent transmission of light, they can be coated on their“exterior” surfaces with a reflective coating such as a thin metalliclayer or other reflective coating. That additional layer will help toensure that layers 212 and 214 do not transmit light. In some instances,however, it may be desirable to provide one or both of the multi-layeredoptical films 212 and 214 as partially reflective to allow some leakageof light, polarized or not, through surfaces 212 and/or 214 in a uniformor other controlled manner. One specific example of a device 210 whereuniform distribution of light is desired is a light guide backlightassembly for a liquid crystal display.

[0105]FIG. 12 is a schematic cross-sectional representation of adiverging wedge optical device 310 according to the present invention.In any optical device employing a diverging wedge design, the opticalsurfaces 312 and 314 are arranged in a diverging relationship in whichthe surfaces diverge as distance from the light source 318 increases. Ina preferred embodiment, the surfaces 312 and 314 are comprised of amultilayer optical film. Also, the optical device 310 is preferablyhollow to minimize absorption losses. It will be understood that, likethe converging wedge device 210, the diverging wedge depicted in FIG. 12could comprise two generally planar optical surfaces 312 and 314 or thatdevice 310 could comprise a generally conical, parabolic or other shapein which the depicted cross-section is taken along the longitudinal axisof device 310. In such an optical device, optical surfaces 312 and 314may actually be portions of a continuous surface which appearsdiscontinuous due to the cross-sectional nature of the view in FIG. 11.

[0106] An optical device which includes diverging optical surfaces willtend to collimate light exiting it as light rays 320 and 322 illustrate.The device 310 depicted in FIG. 12 includes a light source 318 locatedat the entry into device 310. It will, however, be understood that adiverging optical device may include a plurality of sources 318. Ifdevice 310 were formed in a roughly parabolic shape, that collimationwould be more pronounced if the light source 318 was located proximatethe focal point of the parabola. Alternatively, a diverging opticaldevice 310 could also rely on a light generated from a source or sourceslocated away from the actual opening into the diverging optical device310.

[0107] In general, the degree and amount of collimation of light exitingsuch a device 310 is dependent on a number of factors including theangle of light rays entering the device, the location of the lightsource, and the shape and/or angular relationship between the opticalsurfaces 312 and 314.

[0108]FIG. 13 is a cross-sectional schematic view of anotherillustrative optical device 410 formed using the multilayer optical filmaccording to the present invention. The cross-section of device 410 asdepicted in FIG. 13 is taken along a longitudinal axis which shows twogenerally parallel optical surfaces 412 and 414. An additional view isdepicted in FIG. 14, which shows a cross-section of device 410 takentransverse to the longitudinal axis. As depicted, device 410 has agenerally circular shape.

[0109] Optical devices such as device 410 are typically used to transmitlight between two locations and are commonly referred to as “lightpipes.” Such devices have a longitudinal axis and a cross-sectiontransverse to that axis which forms a closed plane figure. Examples ofsome typical cross-section figures include circles (such as that shownin FIG. 14), ellipses, polygons, closed irregular curves, triangles,squares, rectangles or other polygonal shapes. Any device 410 having aclosed plane figure transverse cross-section appears as two surfaces ina longitudinal cross-section as shown in FIG. 13 even though the device410 may actually be formed from a single continuous optical surface.

[0110] Because the multilayer optical film according to the presentinvention used absorbs substantially none of the light incident upon it,light pipes constructed of multilayer optical film according to thepresent invention can extend for a relatively large distances withoutsignificant loss of throughput.

[0111] It is particularly advantageous to use the multilayer opticalfilm with devices such as light pipes in which a large portion of thelight travelling through the device approaches the surfaces of thedevice at shallow angles. Known multilayered polymer reflective filmsare not efficient at reflecting light approaching them at shallow anglesand, therefore, would suffer from large transmissive losses. The presentmultilayer optical film, however, is able to reflect such light with themuch the same efficiency as light approaching the film normal to thesurfaces.

[0112] Alternately, it will be understood that a device such as lightpipe 410 may include sections which are partially transmissive, thusallowing light to escape from the device. The transmission mechanismsmay include multilayer reflective polarizing sections, voids or anyother mechanism as described with respect to the illustrativeembodiments above. Such designs do, however, start to resemble lightboxes or guides depicted and described in conjunction with FIGS. 9 and10, above.

[0113]FIG. 15 illustrates another optical device according to thepresent invention. The optical device 505 depicted in FIG. 15 could beused, for example, in a decorative application such as a flower or abow. Device 505 is constructed of a plurality of multilayer optical filmlayers (such as layers 510 and 520) connected generally in their centersby a post or some other mechanism. Although the layers are depicted asgenerally circular, it will be understood that many different shapescould be provided.

[0114] The layers can be wrinkled or otherwise manipulated to give thedevice 505 volume. The wrinkling of multilayer optical film layers alsoprovides device 510 with multiple converging wedges arranged generallyvertically to return light incident on the device 505 to a viewer.

[0115] Although not required, leakage or transmission of light throughthe layers of multilayer optical film in device 505 is not a greatconcern as transmitted light can be reflected out of the device 505 bythe adjacent diverging wedge formed by the next layer of film. Becauseof the adjacent diverging wedges in device 505, it makes highlyefficient use of leakage between the vertically arranged convergingwedges because light escaping one wedge could be reflected back out ofthe adjacent wedge into which the light is transmitted. As a result,device 505 has an unusually brilliant appearance.

[0116] The multilayer optical film may also be provided in the form ofelongated strips. Such strips of film can be advantageously used to formother configurations of optical devices which can be used, for example,as decorative bows, such as any of those described in U.S. Pat. Nos.3,637,455 (Pearson et al.); 4,329,382 (Truskolaski et al.); 4,476,168(Aoyama); and 4,515,837 (Chong); and in U.S. patent application Ser.Nos. 08/031,560 (Huss) and 08/153,373 (Huss); the entire disclosures ofall of which are incorporated herein by reference.

[0117] Optical device 505 illustrates another significant advantage ofthe optical devices incorporating multilayer optical film according tothe present invention, i.e., that the devices need not exhibit symmetryto be effective. In fact, optical devices according to the presentinvention need not exhibit symmetry in any plane or about any line butcan still function effectively and efficiently due to the low absorbanceand high reflectance both at normal angles and at high angles away fromthe normal of the multilayer optical films.

[0118] Symmetry in optical devices is provided in many instances toreduce or minimize the number of reflections experienced by lighttravelling through the devices. Minimizing reflections is particularlyimportant when using conventional reflectors because of their relativelyhigh absorptivities (see FIG. 8 and the accompanying description above).Because optical devices using multilayer optical film according to thepresent invention experience significantly reduced absorption, it ismuch less important to minimize the number of reflections and,consequently, symmetry is not as important to maintain the efficiency ofthe optical devices.

[0119] As a result, although the illustrative optical devices describedabove do generally exhibit symmetry about at least one axis, the presentinvention should not be limited to optical devices having an axis ofsymmetry. Furthermore, the present invention has been described abovewith respect to illustrative examples to which modifications may be madewithout departing from the scope of the invention as defined by theappended claims.

What is claimed is:
 1. An optical device comprising reflecting means forat least partially reflecting light, the reflecting means comprising atleast one optical surface comprising a multilayer optical filmcomprising: (a) a first layer comprising an oriented birefringentpolymer, the first layer having an average thickness of not more thanabout 0.5 microns; and (b) a second layer of a selected polymer, eachsecond layer having an average thickness of not more than 0.5 microns.2. A device according to claim 1 , wherein the first layer comprises acrystalline naphthalene dicarboxylic acid polyester.
 3. A deviceaccording to claim 1 , wherein the oriented birefringent first layer ismore birefringent than the second polymer.
 4. A device according toclaim 1 , further comprising a plurality of first and second layers,wherein one of the second layers is located between each adjacent pairof first layers.
 5. A device according to claim 4 , wherein the firstand second layers are adhered to each other.
 6. A device according toclaim 1 , wherein the film comprises at least fifty of each of the firstand second layers.
 7. A device according to claim 1 , wherein the filmhas been stretched along a first in-plane axis.
 8. A device according toclaim 7 , wherein the film has a second dimension along a secondin-plane axis generally orthogonal to the first in-plane axis, andfurther wherein the amount of stretch along the first in-plane axis isat least twice the second dimension.
 9. A device according to claim 8 ,wherein the first layers of an oriented birefringent polymer have ahigher index of refraction associated with at least one in-plane axisthan the index of refraction of the second layers of said secondpolymer.
 10. A device according to claim 9 , wherein said higher indexof refraction is at least 0.05 higher.
 11. A device according to claim 9, wherein said higher index of refraction is at least 0.10 higher.
 12. Adevice according to claim 9 , wherein said higher index of refraction isat least 0.20 higher.
 13. A device according to claim 1 , wherein thefilm has been stretched in at least two in-plane directions.
 14. Adevice according to claim 2 , wherein the naphthalene dicarboxylic acidpolyester is a poly(ethylene naphthalate).
 15. A device according toclaim 2 , wherein the naphthalene dicarboxylic acid polyester is acopolyester comprising naphthalate units and terephthalate units.
 16. Adevice according to claim 2 , wherein said second polymer is apolyester.
 17. A device according to claim 16 , wherein said secondpolymer comprises naphthalene units.
 18. A device according to claim 16, wherein said second polymer is a copolyester comprising naphthalateunits and terephthalate units.
 19. A device according to claim 2 ,wherein said second polymer is a polystyrene.
 20. A device according toclaim 2 , wherein said second polymer is a fluoropolymer.
 21. A deviceaccording to claim 2 , wherein said second polymer is a polyacrylate,polymethacrylate, polymethylmethacrylate, or polyolefin.
 22. A deviceaccording to claim 2 , wherein said film has an average reflectivity,for at least one plane of polarization, of at least 50% over at least a100 nm wide band.
 23. An optical device comprising a body having across-section which defines at least two opposing optical surfaces, atleast a portion of at least one of the optical surfaces comprising amultilayer optical film, the film comprising: (a) a first layercomprising an oriented birefringent, the first layer having an averagethickness of not more than about 0.5 microns; and (b) a second layer ofa selected polymer, each second layer having an average thickness of notmore than 0.5 microns.
 24. A device according to claim 23 , wherein thefirst layer comprises a crystalline naphthalene dicarboxylic acidpolyester.
 25. An optical device according to claim 23 , whereinsubstantially all of the optical surfaces comprise the multilayeroptical film.
 26. An optical device according to claim 23 , wherein themultilayer optical film is partially reflective.
 27. An optical deviceaccording to claim 26 , wherein the partially reflective multilayeroptical film comprises a reflective polarizer.
 28. An optical deviceaccording to claim 26 , wherein the partially reflective multilayeroptical film comprises at least one void formed in the multilayeroptical film.
 29. An optical device according to claim 23 , wherein thefirst and second optical surfaces lie substantially in planes which arearranged substantially parallel to each other.
 30. An optical deviceaccording to claim 23 , wherein the body comprises a diverging wedgeoptical device.
 31. An optical device according to claim 23 , whereinthe body comprises a converging wedge optical device.
 32. An opticaldevice according to claim 23 , wherein the body comprises a light pipe.33. An optical device according to claim 23 , wherein the body defines asubstantially enclosed volume and further wherein the optical devicecomprises at least one light source located within the volume.
 34. Anoptical device according to claim 33 , wherein substantially all of theinterior surfaces of the volume comprise the multilayer optical film.35. An optical device comprising a body having a cross-section whichdefines at least two opposing optical surfaces, at least a portion ofeach of the optical surfaces comprising a multilayer optical film, thefilm comprising: (a) a plurality of first layers each comprising acrystalline naphthalene dicarboxylic acid polyester having a positivestress optical coefficient, each of the plurality of first layers havingan average thickness of not more than about 0.5 microns; and (b) asecond layer of a selected second polymer located between adjacent pairsof the first layers, each second layer having an average thickness ofnot more than 0.5 microns, wherein the naphthalene dicarboxylic acidpolyester in the first layers is more positively birefringent than thesecond polymer in the second layers.